The structure and mechanism of action of two membrane-bound ion-transport systems, the (na ion plus K ion)-ATPase from kidney and the CA2ion-ATPase from sarcoplasmic reticulum, are being characterized using nuclear magnetic resonance, electron paramagnetic resonance and kinetic methods. Mn(II) EPR studies of the (Na ion plus K ion)-ATPase and Gd(III) EPR studies of Ca2 ion-ATPase can provide information on the coordination geometry of the bound metal. Water proton NMR studies provide a means of characterizing complexes of the enzymes, metals and substrates or substrate analogues, while FT-NMR methods permit determination of the conformations of enzyme-bound substrates. These structural studies will provide unique and valuable information on the active site structures of these enzymes and the mechanisms of ATP hydrolysis and ion transport.